How much activity can XF handle on a shared server?

Prior to today, my activity peaked each day with between 30 - 50 concurrent users. My site never reached 60 users online at the same time.

Yesterday we published an official interview, it was picked up by a few sources and we have had over 100 concurrent users online for most of the day. Right now we have 70 online. I am on a shared server and everything seems to be running ok.

I wanted to ask the community two questions:

- At what point do you feel it is necessary to move from a shared environment to VPS?

- What is the most activity a XF site has on a shared server, where the page load times seem ok?